This invention relates to an oscillator of an LC resonance type and, more particularly to an improvement in a Colpitts type oscillation circuit.
FIG. 1 illustrates a conventional Colpitts oscillator for use in local oscillation in electronic tuning TV tuners or other applications. This circuit design includes feedback capacitors 11 and 12, a Clapp capacitor 14 and bias resistors 21, 22, 23 and 24 for an oscillating transistor Tr. A diode 31 is used for band switching and especially operates to bring a coil 29 into a shunt state with respect to high frequency components when forward current flows through the diode 31 in a path including a terminal T.sub.2, a resistor 26, the diode 31, a coil 29, a resistor 27 and a terminal T.sub.1. A coil 28 is the only resonance coil which operates under this circumstance, so that the circuit arrangement is ready to receive a high band of broadcasts. A DC voltage is applied between the terminals T.sub.1 and T.sub.2, then to place the diode 31 into a nonconductive state so that both the coils 28 and 29 become operative. The inductance of the resonance circuit thereby increases to be equal to the sum of the inductances L.sub.1 and L.sub.2 of the coils 28 and 29 (i.e. L.sub.1 +L.sub.2). As a result, the circuit arrangement is ready to receive a low band of broadcasts. It is noted that the resonance frequency is determined by the inductance L.sub.1 or L.sub.1 +L.sub.2 together with factors of a variable-capacitance diode 30, a DC cutoff capacitor 15, the Clapp capacitor 14 and the FIG. 2 shows an equivalent circuit of the oscillator circuit of FIG. 1. The inductance L.sub.0 of a coil 32 represents the above mentioned inductance L.sub.1 or L.sub.1 +L.sub.2. A terminal T.sub.0 is a power supply terminal for the oscillating transistor Tr.
A series circuit of the variable-capacitance diode 30 and the DC cutoff capacitor 15, combined with the inductance L.sub.1 or L.sub.1 +L.sub.2, form a tank circuit of a parallel resonance connection. The oscillating frequency is varied with the capacitance D.sub.1 of the variable-capacitance diode 30 which in turn is governed by an applied voltage to a terminal V.sub.T.
Generally speaking, in the case of an oscillation circuit where the resonance frequency is varied with the capacitance C of the tank circuit while its inductance L is fixed, intensity and stability of oscillation are dependent greatly upon frequency due to the frequency characteristics of the oscillating transistor used, the frequency-dependent Q value of the tank circuit and the frequency dependency of the resonance impedance.
Considering the characteristics of the tank circuit, the Q value is represented by ##EQU1## and the resonance impedance by L/cr where r is the equivalent DC resistance of the coil. The Q value and the resonance impedance both decrease with an increase in the capacitance C. Especially when the variable-capacitance diode is used as a factor-varying element, the frequency and capacitance dependent Q value of the variable-capacitance diode would emphase the above tendency for the Q value of the tank circuit and the resonance impedance.
For an oscillator which is used in a local oscillation circuit of an electronic tuning TV tuner and meets the demand for wide range reception in respective bands, for example, VHF and CATV bands (typically, B.sub.1 band: 55-108 MHz, B.sub.2 band: 109-216 MHz and B.sub.3 band: 217-440 MHz, each band encompasses substantially a two-fold frequency width), it is necessary to use a variable capacitance diode which has a width of capacitance variation approximately one and half times as great as that used with a conventinal VHF band receiver.
This necessity results in the wider variation of the Q value and greater difficulties in ensuring constant oscillating conditions throughout all of the relevant frequency bands.